A terahertz electromagnetic wave generator as shown in FIG. 1, which uses polariton mode in a Lithium Niobate (LiNbO3) crystal as a dielectric material, is known as a wavelength-tunable generator of a terahertz electromagnetic wave. In other words, in FIG. 1, a first pump beam (a first pump beam may simply be called “a pump beam”) hv1 and a second pump beam (a second pump beam may simply be called “a signal beam” or “an idler beam”) hv2 generate a terahertz electromagnetic wave by a difference-frequency generation hv3=hv1-hv2 with first and second pump beams entering a Lithium Niobate crystal.
Thus, a coherent light of the terahertz electromagnetic wave hv3 in the frequency band of approximately from 0.7 THz to 2.5 THz is obtained. However, the above-described frequency band is too narrow as a tunable-spectral band for identifying a variety of living materials, by measuring differences in the terahertz electromagnetic wave spectroscopic spectra. By using spectrum of a higher frequency band, the difference of spectrum patterns will become clear so that various molecules can be identified.
When the frequency exceeds 3 TH, because the absorption coefficient of LiNbO3 crystal in a terahertz band becomes extremely large, the output from LiNbO3 crystal decreases significantly, which is one of the causes for limiting frequency band. Moreover, due to the degree of phase mismatching in parallel directions of two pump beams hv1 and hv2 being large, as shown in FIG. 1, the angle matching method must be employed with a large intersection angle between the pump beams hv1 and hv2 at a large output-extraction angle of the terahertz electromagnetic wave hv3. In FIG. 1, terahertz electromagnetic waves hv3 are extracted through a prism array consisting of multiple silicon (Si) prisms 5, which is located on the surface of the LiNbO3 crystal so as to increase the output-extraction angle of the terahertz electromagnetic waves hv3. Therefore, the terahertz electromagnetic wave hv3, the pump beam hv3, and the beam hv3 overlap poorly which causes both frequency band and efficiency to decrease.
On the other hand, it may be also possible that one pump beam enters the LiNbO3 crystal and generates the terahertz electromagnetic wave by a parametric oscillation. In such a LiNbO3 parametric-oscillator-type terahertz generator, by conducting an injection seeding with a continuous wave (CW) laser diode (semiconductor laser), the spectrum linewidth of the generated terahertz electromagnetic wave hv3 can be narrowed to approximately 100 MHz.
In other words, by assigning the LiNbO3 terahertz generator as a slave laser (host laser), and the CW laser diode with narrow spectrum width as a seed laser (master oscillator), the combination of the above two lasers, achieving both characteristics of the slave laser (host laser) and the seed laser (master oscillator), can generate a terahertz electromagnetic wave hv3 with a narrow spectrum width, and then. Using a high stability laser as the seed laser and a high output laser as the slave laser, an oscillation of high stability and high output can be achieved. However, because of the mode hopping of the CW laser diode, on the other hand, it may be difficult to perform frequency-sweep continuously in a wide band. On the other hand, if the injection seeding is not conducted, because the spectrum width becomes extremely wide to exceed 100 GHz (0.1 THz), and the resolution decreases significantly, then it becomes insufficient for the capability to identify substances, in view of a narrow frequency band.